It is known that polyethylene terephthalate may be prepared in the molten state by polycondensation accompanied by liberation of ethylene glycol from prepolycondensates in autoclaves of conventional design. Intrinsic viscosities of approximately 0.7 are usually obtained. Intrinsic viscosities as high as 1.00 can be obtained in specifically designed reactors which improve the evaporation of glycol. These prior art processes have the disadvantage that high reaction temperatures, high catalyst concentrations and long reaction time periods are required to achieve high relative viscosities. The inevitable result is an increasing thermal degradation of the melt which manifests itself by a large number of carboxyl end groups and an intensive yellow coloring of the end product. A high percentage of carboxyl end groups leads to strong signs of degradation during the remelting of the polycondensate. The discolorations are particularly undesirable if pellucid polycondensates are needed for the manufacture of, for instance, packing material for food-stuffs or beverages. In addition, simultaneous with the discoloration, elevated concentrations of aldehydes are to be expected in the melt. These properties are particularly undesirable if the end product is used to manufacture beverage bottles. The aldehydes diffuse into the liquid and affect the flavor of these beverages. Further, such aldehyde containing beverages may be considered to be unfit for human consumption by certain regulatory bodies.
In order to avoid these disadvantages, it is known to produce high viscosity polyesters in the solid phase at temperatures below the melting point of these condensates and under an inert gas blanket or vacuum. The drawbacks of this improvement are the large efforts and expenditure required to prevent the granulate from sticking in the course of the solid state polycondensation.
In one attempted solution to solve this sticking problem, the prior art has subjected the granulate to a continuous mechanically produced movement during the solid state polycondensation. The disadvantage of this prior art process is the need for very long residence time periods for the solid state polycondensation. This requires very expensive reaction equipment with costly agitating elements. In addition, with a continuous process, a very poor residence time spectrum is obtained so that substantial viscosity fluctuations are observed in the finished granulate. This is undesirable for many applications.
Another prior art attempt to solve this stickiness problem is to effect the solid state polycondensation in a fluidized bed in the presence of hot inert gases, such as nitrogen. The necessity of having long residence time periods (which may amount to approximately 20 hours depending on the polycondensability and the molecular weight of the starting material), the desired increase in molecular weight, the particle size of the material, etc., however, requires complicated, expensive equipment and require substantial energy consumption. As in the other prior art processes, the fluidized bed process has an unfavorable residence time spectra. In addition, this fluidized process has the disadvantage of requiring very large quantities of inert gas for fluidization which must be either discarded or subjected to a continous regeneration. This, too, contributes to a very high energy comsumption which is, in many cases, not economically justifiable.
In order to avoid agglomerations due to stickiness the prior art has also added the so-called anticaking agents to the surface of the polycondensed granulate. A plurality of substances which range from fluids to solids are already known as anticaking agents and include solutions, dispersions, emulsions etc. In German Provisional Patent Publication No. 2,117,748, silicon dioxide and silicates, for example, are mentioned as solid or pulverous anticaking agents. It is also known to use glass powder as an anticaking agent. In several product applications it is also necessary to remove the anticaking agent from the granulate upon termination of the polycondensation. This is however possible only if the anticaking agents are soluble. Even if the anticaking agent is soluble, it is necessary to dry the granulate prior to the further treatment.